1. Field of The Invention
This invention relates to a liquid developer for electrostatic photography wherein ether compounds are used as a carrier liquid.
2. Description of The Prior Art
The wet developing system in electrophotography is generally carried out by a process which comprises subjecting a photosensitive material to charging and imagewise exposure to form an electrostatic latent image thereon, developing the electrostatic latent image with a liquid developer which consists of a dispersion of toner particles mainly composed of a resin and a colorant in aliphatic hydrocarbons, transferring the resulting toner image onto a transfer paper sheet, and fixing the image to form a final image.
If a photosensitive paper or film having a photoconductive material, such as zinc oxide, coated thereon is used in the above process, it is possible to omit the transfer step and to directly fix the toner image on the photosensitive material after the development. In addition, the wet development is often used as a developing means such as of electrostatic recording systems wherein an electrostatic latent image is formed on a dielectric material through electric inputting without use of any photosensitive material.
In the wet developing systems, a fine particle toner having a size of from sub-microns to several microns is dispersed, as set forth above, in a carrier liquid having a high electric resistivity, such as aliphatic hydrocarbons. The development of a latent image is based mainly on the electrophoretic principle. This eventually leads to the fact that images with a higher resolution than in dry developing systems making use of toner particles with a size of not smaller than several microns are likely to obtain.
In the two references which Metcalfe reported at his initial stage, i.e. (K. A. Metcalfe, J. Sci. Instrum., 32, 74 (1955) and ibid., 33, 194 (1956), it is stated that a great number of organic or inorganic pigments including carbon black, magnesium oxide and the like are usable as pigments of liquid developers and gasoline, kerosine, carbon tetrachloride and the like are usable as a carrier liquid.
In Japanese Patent Publications issued at the time corresponding to the early stage of Metcalfe, there are stated the use of halogenated hydrocarbons as the carrier liquid (Japanese Patent Publication No. 35-5511), and the use of polysiloxanes (Japanese Patent Publication No. 36-14872) and ligroin and mixtures of these petroleum hydrocarbons (Japanese Patent Publication Nos. 38-22343 and 43-13519).
Among patent publications which are directed to processes of making toners, there are a number of patent publications which deal with carrier liquids. Typical examples include Japanese Patent Publication Nos. 40-19186, 45-14545 and 56-9189. The carrier liquids (which may also serve as a dispersion medium at the time of polymerization) set forth in these publications include aromatic hydrocarbons such as benzene, toluene, xylene and the like, esters, alcohols, and aliphatic hydrocarbons such as n-hexane, i-dodecane, Isoper H, G, L and V (Ekson Chem. Co., Ltd.)
However, the hitherto proposed carrier liquids are mostly composed of organic solvents whose vapor pressure is high. This leads to the following problems: i) the vapor of carrier liquids discharged at the time of fixing is liable to cause environmental pollution; and ii) the vapor is very likely to cause ignition.
To cope with the above problems, it may occur that in order to lower the vapor pressure of carrier liquids, petroleum solvents such as low vapor pressure hydrocarbons are used as the carrier liquid. If the molecular weight of hydrocarbons is increased so that the vapor pressure is lowered, the carrier liquid using such hydrocarbons is increased in viscosity, thus adversely influencing the developing speed. Moreover, since the melting point of the carrier liquid increases to the neighbourhood of room temperature, it becomes necessary to invariably heat the carrier liquid for use as a liquid developer. This is unfavorable from the standpoint of energy saving, thermal pollution and deterioration of the liquid developer.
In Japanese Laid-open Patent Application No. 51-89428, there has been proposed the use, as the carrier liquid, of hydrocarbon solutions which have an electric resistivity of not lower than 10.sup.9 .OMEGA..multidot.cm and a dielectric constant of not higher than 3.0. Hitherto proposed carrier liquids are predominantly composed of non-polar hydrocarbon solutions which have a high electric resistivity and a low dielectric constant. It is empirically known that if the electric resistivity of the carrier liquid is lower than an appropriate level, a latent image on a photosensitive material may be broken, or a bias leakage at the developing and transfer units may take place, not resulting in images of good quality.
The liquid developers which contain non-polar carrier liquids having high electric resistivity and low dielectric constant are not always satisfactory with respect to the chargeability to toner and its stability in relation to time. More particularly, there arise the problems that the charge quantity of toner is reduced as time passes and that the quantity of a reverse polarity toner is increased.
Thus, there have never been obtained any carrier liquids which are satisfactory for use in hitherto proposed liquid developers.